Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design.

The broken lines shown in the FIGURE represent portions of the display screen or portion thereof with a graphical user interface that form no part of the claimed design. 

CLAIM I claim the ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 